Discussion in 'Electronic Basics' started by philips, Jun 21, 2004.

1. ### philipsGuest

Gain is usually V2/V1, but sometimes it's I2/I1, or even V2/I1
(also known as Z21) I was simply wondering when it is appropriate
for gain to be V2/V1 or I2/I1 etc i.e. is it more informative
to talk about voltage gain than current gain, is it hourses for
courses?

2. ### John PopelishGuest

It depends on what is important in the circuit. If it is a signal
amplifier and the signals are represented by voltages (as in common
with audio) and input impedance is assumed to be much higher than the
impedance of the source feeding the stage, and the output impedance of
the stage is much lower than its load, voltage gain is the natural way
to look at the stage. If the stage is an RF gain stage that needs to
match source and load impedances, then voltage gain and current gain
are related by the pair of impedances, so voltage gain, current gain ,
Vout/Iin, Iout/Vin, or most useful, PowerOut/PowerIn are all
reasonable. If the voltage gain is nearly 1 as with all followers,
voltage gain is not nearly as useful a way to describe the stage as
current gain.

Am I addressing your question or did it go right by me?

3. ### CFoley1064Guest

Good question. Horses for courses. If you're talking about a standard op amp,
you usually talk about voltage gain. For a current-controlled device like a
standard transistor, you usually talk about current gain. For amplifiers, you
frequently talk about power gain. All three are common, but it's always
apples2/apples1 or oranges2/oranges1. V2/I1 might measure impedance, but it
doesn't quantify gain. Gain of any kind is a dimensionless quantity, which
means the terms have to cancel.

Gain is sometimes quantified logarithmically in decibels. Here's a good basic
tutorial on decibels from the National Instruments Developer Zone:

http://zone.ni.com/devzone/conceptd.nsf/webmain/36EA112B7F683D768625681100
4DD454

Good luck
Chris

4. ### philipsGuest

Hi, thanks for the reply. I have another question (maybe slightly
more complicated). When we learnt about Network Synthesis, the
lecturer always talked about transfer functions. Is gain and
transfer functions the same thing. Also, a lot of the time my
notes refer to Z21 transfer function, is this gain too, i.e.
V2/I1

5. ### John PopelishGuest

Gains described as V/I have units of ohms, usually referred to as
transresistance. Gains of I/V have units of siemens (reciprocal ohms)
usually referred to as transconductance. You have to combine these
ratios with source or load impedances to make much sense of them as
gain. A transformer can produce a wide variety of transresistances or
transconductances in a given circuit but none of them provide power
gain.

6. ### JeffMGuest

[gain is] always apples2/apples1 or oranges2/oranges1.
Probably because he meant I2/V1 (transconductance).

7. ### Bob MyersGuest

Nope; if I2/V1 is a valid sort of "gain" (and it is), then
so is V2/I1 (transresistance, as John said - the shortened
version of "transfer resistance"). And just what do you
think is the origin of the name "transistor"?

Just because you're not familiar with "gain" being expressed
in ohms doesn't mean it's not a valid thing to do - and actually,
pretty useful in the proper context.

Bob M.